Pressure-sensitive adhesives find broad utility in skin-contacting medical applications providing the means for reversibly securing tapes, dressings, and devices to the patient as detailed in "Porous and Other Medical Pressure Sensitive Adhesives", K. Krug and N. M. Marecki, Adhesives Age, p. 19, November 1983 and "Hospital and First Aid Products", D. Satas and A. M. Satas, Chapter 25 in Handbook of Pressure Sensitive Adhesive Technology, Second Edition, D. Satas, Editor, Van Nostrand, Rheinhold, 1989.
An adhesive composition can also serve to adhere a biomedical electrode to skin and to establish an electrical connection between skin and an electrical medical apparatus desirably has multiple characteristics that are difficult to achieve in one composition. The composition should have the characteristics of a good medical adhesive and those of a good electrical conductor. Electrical conductivity is imparted by ionic species in polar adhesive compositions.
Ionically-conductive, pressure-sensitive adhesives suitable for use in many biomedical electrodes are shown in many patents. U.S. Pat. No. 4,524,087; U.S. Pat. No. 4,539,996; U.S. Pat. No. 4,554,924; U.S. Pat. No. 4,581,821; U.S. Pat. No. 4,674,512; U.S. Pat. No. 4,777,954; U.S. Pat. No. 4,684,558, and U.S. Pat. No. 4,848,353 are exemplary.
Ionically-conductive adhesives made according to the above listed patents are used in patient grounding plates, transcutaneous electrical nerve stimulation (TENS) electrodes, and diagnostic electrocardiogram (EKG/ECG) electrodes. While these adhesives provide adequate adhesive and electrical properties for some applications, optimizing the electrical properties without adversely affecting adhesion properties has been difficult. It is known that increasing the water content of the adhesive compositions described in the above mentioned U.S. patents to about 25% improves the electrical performance of electrodes coated with the adhesives. The reason for this empirical observation is not known. One possible explanation is that the increased water facilitates hydration of skin, thereby reducing skin impedance. Unfortunately, increasing water content to optimum levels for electrical performance is found to decrease the tack of the adhesive, resulting in lower skin adhesion.
Another ionically-conductive adhesive is disclosed in GB-A-2,115,431 and U.S. Pat. Nos. 4,699,146 and 4,750,482. The adhesives described therein are formed by dissolving or dispersing polymers in a plasticizing liquid and subjecting the mixture to radiation energies at least equivalent to 100,000 electron volt (X-ray, gamma and beta ray, and electron beam irradiation). Present with the polymers are irradiation-compatible, nonvolatile elasticizers that among others include mono- or diethers of a polyalkylene glycol, mono- or diesters of a polyalkylene glycol, and an imidazoline derivative amphoteric surfactant. But cautions are also provided not to permit plasticizers containing surfactants or detergents from contacting skin, in order to assure that the adhesive is hypoallergenic and not skin irritating. These patents also fail to recognize the beneficial effect of having such surfactants or detergents present in a polar adhesive to allow for absorption of skin oil away from the adhesive/skin interface providing better bond formation. Indeed these patents caution that such materials are better employed so as not to be in contact with skin, in order to assure that skin irritation is not caused.
Mammalian skin naturally exudes a variety of oils and other lipophilic compounds that protect the skin. Human skin surface lipids are produced both by sebaceous gland activity and by the epidermis as noted in "Sebaceous Glands", John S. Strauss, Donald T. Downing, and F. John Ebling, p. 569, Chapter 26 of Biochemistry and Physiology of the Skin, Lowell A. Goldsmith, editor, Oxford University Press, New York, 1983. Areas rich in sebaceous glands have a higher total lipid concentration, ranging in one study from a high of 160 micrograms/sq cm on the forehead to 19 micrograms/sq cm on the leg. Moderate levels are seen on the chest (59 micrograms), side (29 micrograms) and arm (30 micrograms) which along with the leg are areas of application of the biomedical electrodes which are one of the uses for the adhesive of the present invention. The composition of these lipids varies somewhat dependent on the relative contributions of these two sources with areas rich in sebaceous glands having higher concentrations of wax esters and squalene. Major components in the skin surface lipids include triglycerides (average 30%), free fatty acids obtained from bacterial hydrolysis of the triglycerides (average 30%), wax esters (20%), and squalene (10%). The fatty acids that make up the triglycerides are predominately linear saturated C16 (palmitic acid, 24%), mono-unsaturated C18 (oleic acid, 36%), mono-unsaturated C16 (9%) and linear saturated C18 (stearic acid, 8%) according to C. Carruthers, in Biochemistry of Skin in Health and Disease, Chapter 4, "Lipid Composition", p. 73, Charles C. Thomas Publisher, Springfield, Ill., 1962. This layer of skin surface lipid serves as a weak boundary layer, limiting the ability of a pressure sensitive adhesive to rapidly for a bond with the skin. A pressure sensitive adhesive, such as an acrylate or rubber-resin based material, may absorb the skin oil away from the interface allowing for bond formation. Polar adhesives, such as those ionically conductive adhesives cited above, tend to be incompatible with the oil, leading to surface detackification and poorer bond formation.